The present invention relates to DC-to-AC converters for electric power systems. Lower cost, high-power, efficient, DC-to-AC converters are of interest for solar energy economics. In the prior art, DC-AC inverters are the second highest cost item next to the photovoltaic panels. For high efficiency and low heat dissipation, commutation of DC to produce AC preferably uses solid state switches that are either fully on or fully off, and do not dwell more than a microsecond or so in an intermediate state. Therefore it is more complicated to produce a sine wave that takes on all values between the negative peak and the positive peak. On the other hand, producing a square wave which switches between the positive peak and the negative peak produces a form of AC that is not suitable for all loads.
Various manufacturers provide prior art DC-AC converters, that fall into one of a few broad classes and operating modes. The class of “modified sine wave” converters maintains both the same rms and the same peak voltage as a sine wave, while still employing only on-off commutation. This is done by switching the voltage between the desired positive peak, zero and the negative peak, spending 50% of the repetition period at zero, therefore achieving both the same peak and the same rms values as a true sine wave, and being compatible with a greater variety of loads.
Still, there are loads that do not tolerate the modified sine wave; for example appliances that present inductive loads, such as induction motors, some cellphone and laptop battery chargers, fluorescent lamps and tumble dryers, and any device with an internal power supply that uses capacitive reactance as a lossless voltage-dropping means, can malfunction on modified sine waveforms. Moreover, there is a potential problem with radio and TV interference due to the high level of harmonics of the modified square wave converter. Such a waveform is therefore not a candidate for coupling solar-generated power into the utility network or into house wiring.
“True sine wave” is another class of prior art DC-AC converter. Linear amplifiers provide the absolute cleanest AC power waveforms, but their inefficiencies cause high heat dissipation in converters of several kilowatts capacity. Moreover, linear amplifiers lose efficiency rapidly when operating into non-unity power factor loads. Some sine wave inverters overcome the problems with linear amplifiers by using digitally-synthesized waveforms, which are multi-step approximations to a smooth sine wave. One example of a step-approximation sine wave inverter is the XANTREX (formerly Trace) SW4048.
In U.S. Pat. No. 5,930,128 by current Inventor, a power waveform generator was disclosed that involved expressing the sinusoidal waveform as a series of numerical samples in a number base comprising a plurality of digits; selecting corresponding digits from each numerical sample and generating therefrom a waveform corresponding to the sequence of each digit, then using combining means to form a weighted combination of the digit-corresponding waveforms, the weights being chosen in relation to the numerical significance of each digit. For example, using a ternary number base, the weighting means would add the digit waveforms in the ratios 1:1/3:1/9 and could for example be a transformer with these turns ratios.
U.S. Pat. No. 5,373,433 also describes using series connected, turns-ratio weighted transformer coupling of 3-level waveforms to produce a 27-level step approximation to a sine wave. The principle described therein is similar to that used in the aforementioned XANTREX SW4048 inverter. The combining means disclosed in the '128 patent for combining digit-corresponding waveforms was, in a low-frequency case, a series connection of transformers having turns ratios in the ratios of corresponding numerical digits, and in a high-frequency case, comprised a set of quarter wave lines having characteristic impedances in the ratios of corresponding digits.
In a device built in accordance with the '128 patent, the series-connected transformer is the appropriate version for 60 Hz, as ¼ wavelength lines are impractical at 60 Hz; however, the transformers needed for the inventions of the '128 and '533 patents represent a significant fraction of the total cost and weight of medium-power converters, and also account for a few percent loss in total efficiency. Therefore, other solutions that avoid the disadvantages and pitfalls of the above prior art would be useful, and in particular, a solution avoiding these low-frequency transformers would be a benefit.
Transformerless inverters are known in the prior art, particularly for utility-interactive inverters, which use high-frequency switching or pulse width modulation to approximate a sinewave. However, a disadvantage that arises in these prior art converters is the imposition of the high-frequency switching waveform on the solar array, which can capacitively couple through the glass cover upon touching it, potentially causing RF burn to personnel or damage to the solar panel, as well as causing the solar array to radiate substantial radio interference. Thus a design is required that can create a more benign voltage fluctuation on the solar array DC conductors.
Another categorization of convertor relates to whether they are designed to power loads directly, or whether they are designed to feed and sell power back into the electricity grid. A load inverter that can power loads directly is said to operate in standalone mode, and is also called a “standalone inverter”, while a grid-tie inverter is said to operate in grid-interactive mode and is also called a “grid-interactive inverter”.
For safety and other reasons, the latter have to meet different specifications than the former, especially under fault conditions. In particular, a load inverter should be a constant voltage source, while a grid-tie inverter does not have a constant voltage output but must adapt to the voltage of the grid, and is a current source. Moreover, a load inverter is always used with battery storage, and should maintain efficiency at both light and heavy loads and have low no-load power consumption, so that the battery is not discharged while the inverter is idling at night. Grid-tie inverters however do not have the same a requirement for no-load power consumption, as they do not operate at night.
A complete alternative energy installation may thus comprise a number of functions, including load inverters, grid-tie inverters, load management for manually or automatically transferring load between the utility and alternative energy supplies, storage batteries, battery chargers, circuit breakers, surge protectors and other safety devices to protect equipment and wiring and eliminate the risk of electrical mishaps under conceivable fault conditions. Other than the inverters and the array these additional components are known as “balance-of-system” components.
For high power grid-tie installations, typically 20 kw and above, 3-phase inverters are preferable in order to keep the gauge and cost of wiring down and to assist in maintaining balance between the three phases of the electricity grid. For converters over 100 Kw, 3-phase is often mandated by the utility company. Three phase inverters using pulse width modulation are known from the art of solid state Motor Drives, but they are not suitable for grid-interactive use for many reasons, and Motor Drives do not need or have ground leak detection on the DC bus, which is internal.
The total cost of balance-of-system components required in an installation can be significant; therefore it is an objective of this disclosure to describe novel designs of inverters, safety devices and automatic load management devices that provide a more efficient and cost effective complete installation, and which achieve cost reductions in the electronics to complement the currently falling cost of photovoltaic panels.